Multi-spindle machine-tools with multiple workpiece spindle units are, for example, known from EP 0 726 119 A1 or DE 196 19 720 A1. These known machine tools have a spindle drum, mounted on a machine frame, on which the workpiece spindles are mounted at rigidly fixed equal angles to one another and with which they are brought into the various machining positions by advancing the spindle drum. When the spindle drum is arrested in order to fix the workpiece spindles in their respective machining positions, machining of the workpieces clamped into the workpiece spindles takes place at the machining points, after which the spindle drum is advanced again and the workpiece spindles with the workpieces reach their next machining position.
It is a known practice, from DE 196 19 720 A1, to provide the individual machining stations with tool slides with two tools and to advance the spindle drum in such a way that, within each machining station, each spindle can be positioned successively in two positions so that the workpieces clamped in them are first machined by the first tool--in the direction of travel of the spindle drum--and are then, after advancing by a small angle, machined by the second tool. In this machine, twice as many machining positions as workpiece spindles are provided, if one looks down from the loading and unloading stations for loading and unloading of the workpieces.
The regular disposition of the workpiece spindles on the spindle drum in the known multi-spindle machine-tools has the disadvantage that the spindle drum can only be advanced when the longest-lasting machining process at one machining station has been completed. The stroke-time, that is the time until the drum is advanced, along with all workpiece spindles, is thus determined by the longest lasting machining operation at one machining station. It is thus often the case that only one tool at its respective station is engaged upon the workpiece, while at all other machining stations processing is already complete and the machining capacity of the individual machining stations cannot be exploited fully because of the enforced dead time. A further disadvantage of the advanceable spindle drum is that, for it to be moved in this circular advance process, its mass should ideally be as small as possible in order to achieve the required high speed of movement to advance the workpiece spindles into the next machining position. In order to keep the mass of the spindle drum within reasonable limits, its diameter and thus also that of the spindle circle must be kept as small as possible, which restricts the room available for the workpiece spindle units and which also leaves little room for arrangement of the machining tools in the individual machining stations and for the loading device. Reducing the spindle drum to the smallest possible size means that the central spindle-carrier too, on which the spindle drum, especially in the case of vertical machines, is normally mounted, cannot be designed as large as one would like so that it often lacks the required rigidity. A further disadvantage of the known machines is that the tool-carriers are arranged in fixed positions and thus cannot always be positioned optimally with respect to the workpiece.